contrarian (n.)

"a contradictor, one that is by nature in opposition to prevailing  opinions, or the shibboleths of the majority," 1963, from contrary + -ian.

To be in opposition is not to be a nihilist. And there is no decent or charted way of making a living at it. It is something you are, and not something you do. [Christopher Hitchens, "Letters to a Young Contrarian," 2001]

Latin contrarius (adj.) also was used as a noun meaning "an opponent, an antagonist." In English history, contrariant (from French, from Medieval Latin contrariantem) was the name given to Thomas, Earl of Lancaster, and the barons who took part with him in the rebellion against Edward II, "because, on account of their great power, it was not expedient to call them rebels or traitors" [Century Dictionary].